1. Field of Invention
This invention relates to a photodetector module which is utilized in optoelectronic communication systems and a method of making the photodetector module. The photodetector module is a device which receives optical signals propagating in an optical fiber and converts the optical signals into electric signals. A photodetector module is produced by mounting a photodiode chip, a preamplifier chip and a capacitor on a package, sealing the package with a cap having a window and sticking a sleeve above the cap to the package. The sleeve is a cylindrical receptacle in which an end of an optical fiber will be inserted. A lens is often fitted on the window of the cap In order to converge the light beams emitted from the end of an optical fiber on the photodiode chip. The device is called a "photodetector" module, because the device has a photodiode which converts optical signals to electric signals and a preamplifier for amplifying the electric signals in the same package.
2. Description of Related Art
This application claims a priority based on Japanese Patent Application No. 126973/1994 filed 16 May, 1994. Prior photodetecting device was usually built of a package and a photodiode chip mounted on the package. Optical signals are converted to electric signals by the photodiode and are output from the device. The electric signals are amplified by an preamplifier which is furnished on a print circuit plate out of the photodetecting device. The outer-installed preamplifier, however, cannot prevent external noise from mixing with a weak signal, since a weak signal propagates in a long, narrow wire. In the case of a high frequency signal, the self-induction L of the long wire deforms the electric signal. Contrivance which furnishes a photodiode and a preamplifler on close positions in the same package has been suggested in order to suppress the bad influence of the external noise. For example, the following module was proposed;
1 NOBUYOSHI HORIGOME, KAORU MORIYA, IWAO ODA, EIJI KIKUCHI & SEISHI CHIBA, "Pre-Amp containing PD module", 1990 Spring Conference of Electronics Information and Communication Society, the 4th volume, C271, p4-326 ( FIG. 3 of this application shows the structure of the PD module) PA0 1 YOSIKAZU SAWAI, KAORU MORIYA, MASAHIKO HIRAO & HIROO KITASAGAMI, "Pre-Amp containing PD module", 1993 Spring Conference of Electronics Information Communication Society, 4 band, C-186, p4-222 (FIG. 4 shows the device).
The proposed PD module has a stem (30), a plate (34) fixed on the stem (30), a photodiode and a preamplifier which are mounted together on the plate (34). A cap (31) is positioned and is welded on the stem (30) by a laser. A lens (32) has been fitted in an opening of the cap (31). A receptacle (33) is also positioned and is welded on the front end of the cap (31). The photodiode is an InGaAs-PIN photodiode. The preamplifier is a Si bipolar transistor type amplifier. The PD module adopts an optical fiber with a core of a 50 .mu.m diameter and a cladding of a 125 .mu.m diameter. This module has a diameter of 10 mm and a length of 14 mm. The report writes that the -3 dB attenuating frequency is 200 MHz. The author describes that the device obtained a flat frequency performance without peaking and the necessary properties for an optical network of a 100 Mbit/sec band. The device has a photodiode and a preamplifier mounted on a print plate. The electrode pads and patterns are connected by bonding wires between them. The purpose of the PD module is to enhance the speed of response and reliability of the device by assembling the preamplifier in the package together with the photodiode.
The device is produced by mounting a photodiode and a preamplifier on a plate (44) fixed on a stem (40), positioning a cap (41) having a lens and fixing the cap (41) on the stem (40). The device has a receptacle (holder) on the cap (41). An end of a fiber assembly (42) is inserted into the receptacle. A nut prevents the fiber end from being plucking out of the receptacle. The preamplifier is a Si bipolar transistor type. The photodetector is an InGaAs-PIN photodiode with a light-receiving region of an 80 .mu.m diameter. In the case of making a device of high sensitivity, the device is built with a GaAs-FET preamplifier and an InGaAs-PIN photodiode with a light-receiving region of a 50 .mu.m diameter. The optical fiber is a single-mode fiber with a core of a 10 .mu.m diameter and a cladding of a 125 m outer diameter. The module has a 7 mm diameter except for the flange and a 24 mm length.
These improvements try to cut the noise commonly by mounting a photodiode and a preamplifier together in a package and shortening signal propagating wires. These photodiode modules use a special IC (integrated circuit) for the preamplifier of the signal of the photodiode. FIG. 3 and FIG. 4 do not clearly depict the way of mounting the preamplifier and the photodiode on the package. Thus, two examples will be described for clarifying the way of mounting the photodiode and the preamplifier.